The present invention relates to a method of static reactive power compensation for maintaining the system voltage of a power supply system.
A conventional apparatus for effecting a method of the type described is shown is FIG. 1. Designated at 1 is an infinite bus; 2, a booster transformer; 3, a power transmission line (which may for example be a single-circuit 200 km, 500 kV transmission line); 4, a generator (which may for example be a generator having a total capacity of 2,500 MW); 5, 9 booster transformer; 6, a feeder high-tension bus (at 500 kV, for example); 7, an intermediate bus (at 500 kV, for example); 8 and 9, two-circuit power transmission lines (which may for example be 100 km, 500 kV lines); 10, a fault point in the power transmission line 9, and 11, 12, transmission line circuit breakers. A static reactive power compensator (hereinafter referred to as an "SVC", which is short for `static var compensator`) comprises a voltage-reading transformer 14, a capacitor 15 for supplying advanced-phase reactive power, a reactor 16 for supplying lagged-phase reactive power, and 17 is a thyristor switch for changing the current flowing through the reactor 16.
The SVC 13 further includes a transformer 18 for detecting the voltage at the intermediate point, a voltage detector circuit 19 for rectifying the output signal from the transformer 18 into an analog signal, a compartor circuit 21 for comparing the analog signal from the voltage detector circuit 19 with a reference voltage signal 20, a voltage control compensator circuit 22 for stabilizing the voltage control operation within the SVC 13, and an ignition circuit 23 for supplying trigger signals to the thyristor switch 17.
The operation of the apparatus thus constructed will now be described. The SVS 13 is connected to the intermediate bus 7 for supplying advanced-phase or lagged-phase reactive power to the power transmission lines to maintain the voltage at the intermediate bus, thus improving the stability of the power supply system under varying conditions. Assuming that the capacitor 15 has a capacity of 1,000 MVAR and the reactor 16 and the thyristor switch 17 have a capacity of 2,000 MVAR, the reactor 16 is rendered variable in capacity in the range of from 0 to 2,000 MVAR so that the SVC 13 can supply reactive power which is continuously variable from 1,000 MVAR with an advanced phase to 1,000 MVAR with a phase lag.
The transformer 18 serves to detect the voltage at the intermediate bus 7. When the voltage at the intermediate bus 7 is lower than the reference voltage signal 20, the SVC 13 supplies advanced-phase reactive power. When the voltage at the intermediate bus 7 is higher than the reference voltage signal 20, the SVC supplies lagged-phase reactive power to maintain the intermediate bus voltage at the reference voltage.
FIGS. 2 and 3 are illustrative in more detail of such voltage control operation.
In FIG. 2, the reactor 16 is cut off by the thyristor switch 17 with only the capacitor 15 connected in an interval from 0 to A, in which the advanced-phase reactive current varies in proportion to the voltage at the intermediate bus 7. During an interval from A to B, a constant-voltage characteristic is maintained under the control of the thyristor 17. The thyristor switch 17 is fully closed in the interval from B to C, during which the lagged-phase reactive current varies with the voltage. The slope of the line A--B is determined by the relationship between a signal indicative of the deviation of the reference voltage signal from the intermediate bus voltage and the signal supplied to the ignition circuit 23, that is, the control gain K of the voltage control compensator circuit 22. The slope is normally selected to allow the reactive power to vary from 0 to 1,000 MVAR with a voltage fluctuation ranging from 3 to 5%.
FIG. 3 shows a simplified relationship between the power supply system and the SVC. As viewed from the intermediate bus 7 to which the SVC 13 is connected, the power supply system can be regarded as a voltage source 30 having an impedance 31 (L.sub.v), the voltage source 30 and its impedance 31 being variable from time to time.
When the voltage of the voltage source 30 is V.sub.01 which is equal to the reference value, the operation of the SVC is at a point a in FIG. 2, whereupon the SVC produces an output of zero. When the voltage of the voltage source 30 drops to V.sub.02, the SVC supplies advanced-phase reactive power, and the intermediate bus voltage increases along a slope dependent upon the impedance 31, namely, the curve 2, so that the operating point of the SVC is shifted to point b, with the voltage at the intermediate bus 7 being maintained in the vicinity of V.sub.01. When the voltage of the voltage source 30 is further reduced to V.sub.03, the power supply voltage characteristic is governed by the curve 3, so that the operating point of the SVC drops to the point c, which is below the point A. At point c, the reactor 16 is completely open and the capacitor 15 is connected in the circuit, but the SVC 13 is incapable of maintaining a constant voltage. Therefore, the voltage at the intermediate bus 7 undergoes a fluctuation which is substantially the same as the voltage drop from V.sub.02 to V.sub.03.
The stability of the electric power in a power supply system reaches its limit when the phase angle between the voltages at the sending end and the receiving end is 90 degrees. With long-distance power transmission, the phase angle is 30 to 40 degrees, at the most, with an internal impedance of the generator being removed. Where the SVC is installed to maintain the intermediate bus voltage, the phase angle between the voltages at the sending end and the intermediate bus, and the phase angle between the voltages at the intermediate bus and the receiving end are 90 degrees, respectively, at the most, and hence the phase angle between the sending and receiving ends can be around or greater than 90 degrees to thereby increase the maximum electric power that can be transmitted.
By maintaining the intermediate bus voltage using the SVC, the stability in the transient state can be improved as follows: FIG. 4 is illustrative of the operation of an unstable power supply system similar to that shown in FIG. 1, but having no SVC, and FIG. 5 shows operation of a stable power supply system with an SVC installed at the intermediate bus. FIGS. 4 and 5 show waveforms recorded of system fluctuations produced when the circuit breakers 11, 12 are opened four cycles after a three-wire ground fault has occurred at the intermediate point 10 in the transmission line 9. A study of FIG. 4 indicates that when the faulty transmission line is severed by the circuit breakers 11, 12, the voltage at the intermediate bus does not return quickly to a steady voltage value as the voltage drop across the transmission line is increased. Therefore, the electric power supplied by the generator 4 to the power supply system is reduced. Since the mechanical input to the generator 4 does not change rapidly, however, an accelerating force acts on the rotor of the generator to progressively increase the phase angle of the generator 4. When the phase angle is excessively advanced, the electric power supplied from the generator 4 to the power supply system is increased to the point where it exceeds the mechanical input to the generator 4. The phase angle of the generator 4 begins to be reduced, and the output electric power from the generator 4 begins to be reduced about 1 second after the fault has occurred. The phase angle of the generator 4 is thus caused to fluctuate with a period of on the order of 2 seconds. In this condition, the system may exceed stability limits and be thrown into an unstable condition in which the generator 4 is subjected to step-out.
In FIG. 5, when the voltage drops due to the threewire ground fault, the SVC 13 supplies the system with advanced-phase reactive power at about 800 MVAR to maintain the intermediate bus voltage at a substantially steady voltage level. The electric power supplied from the generator 4 to the system is kept at a constant level without being reduced, so that no accelerating force acts on the rotor of the generator 4 and hence the phase angle thereof will not be increased or reduced. Accordingly, fluctuations in the power supply system can be held to a minimum. The SVC thus serves to improve the stability of the system under transient conditions.
When the SVC operates at point b in FIG. 2, the SVC has already supplied the system with advanced-phase reactive power approximating the maximum capacity. Even when the system undergoes a ground fault, the SVC's capability to maintain the voltage of the system may be exceeded, and the voltage at the intermediate bus 7 drops to the point c, with the result that the system oscillates and becomes unstable. Since the voltage at the intermediate bus 7 in the steady state varies from time to time with the voltage of the generator 4, the condition of the transmission lines, and the electric power transmitted, the above difficulty can arise when the reference voltage value is set at the constant level V.sub.01.
With the conventional SVC thus arranged, when a transmission line suffers a ground fault while electric power is transmitted with the intermediate bus voltage being at a level lower than a reference voltage, the SVC cannot maintain the voltage, and the generator undergoes step-out.
Recently, a further static compensation system has become known, as described in "Control of Shunt Compensation-Viewpoint Two" by Romegialli et. al., presented at the London IEE seminar, September 1980. Both fixed capacitor and thyristor switched capacitor systems are analyzed therein, and a control system is proposed wherein the VAR production of the SVS is controlled by correcting the voltage set point. The reactive power is compared with range limits and switching signals, which may be suitably delayed, are accordingly output. However, this system requires a VAR transducer for giving a value indication to the VAR regulator, and other components leading to an overly complex control function. In the present invention, advantageous effects can be derived from the use of a simple delay filter device, largely simplifying the apparatus performing the control function.